Devulcanization is a method of recycling waste cross-linked rubber, such as scrap rubber for molding processes, to produce a material that can be revulcanized into useful products. Known processes for devulcanization are chemical, mechanical, ultrasonic, biological using microorganisms, or use microwaves or steam.
One such mechanical process is known as high shear mixing. This process provides extremely high levels of shear and extensional stressing thereby inducing chemical reactions in the material being processed. U.S. Pat. No. 7,461,970, assigned to the applicant, describes such a process. It comprises a mixing apparatus having two opposing mixing members rotatable relative to one another about an axis which have facing surfaces extending away from the axis and which define a mixing chamber therebetween. An array of mixing formations on at least one of the surfaces interact to mix the material and propel the material within the chamber toward the axis. This mechanical process has been demonstrated to have the ability to un-crosslink thermoset elastomers using shear energy such that approximately 60% of the original physical properties of the materials are recovered.
The material produced at the output of the high shear mixer still has some cross-linking, so it is sent for post processing in a roll mill. This consists of two horizontally arranged rollers running at slightly different speeds. Feedstock from the high shear mixer is loaded into the mill and as it passes between the rollers it is subjected to a shear force, which further breaks down the cross-linking bonds and produces a mixture of granules dispersed within a fluidized material. The material passing through the rollers is recycled to the top, and generally subjected to about 17 passes, after which it forms a sheet like material, which can then be used as a source of primary rubber for the original molding process. This process thus recovers material that would otherwise be wasted and allows it to revulcanized and made into useful products. While effective, it is batch operated since the material loaded into the roll mill has to be passed through the nip multiple times.